It can keep the response of the mid driver smoother. . . usually
the port will split the resonant peak into two smaller
peaks keeping the response smoother and gives the driver
a better roll-off at the bottom end. . .

The best type of sub + main speaker systems have the mains that
can produce the low frequencies well. . .they usually have a smoother
roll-off and sound more natural. . .the mains are still going to be
getting some of the frequencies due to the cross-over slope being
12 dB/Oct or 18 dB/oct or 24 dB/oct or higher but it is still not
an infinite brick wall that separates the two. . .both the sub and
the mains need to be able to produce the frequencies two or
three octaves beyond the crossover point. . .

A ported alignment will create a 18db/oct (3rd order) acoustic filter below the tuning frequency, a sealed enclosure will create a 12db/oct (2nd order) acoustic filter below the tuning frequency. Either of these can be used along with electric filters set at or near the tuning frequency to create a steeper effective acoustic filter.

In my personal experience, I find it easier to blend a sealed enclosure with a subwoofer.

There are two other advantages of a sealed enclosure that may or may not be important to you.

1. They are usually physically smaller.
2. If you choose not to run a high pass filter to the mains (not recommended), they provide better protection against large excursions at low frequencies. A ported box does not substantially load the woofer below the tuning frequency and will provide little protection against over excursion of the woofers.